Tungsten-tantalum alloy



Patented Dec. 31, 1929 UNITED STATES PATENT OFFICE JOHN HUMPHREYS RAMAGE, 0F BLOOMFIELD, NEW JERSEY, ASSIGNOR T0 WESTING- HOUSE LAMP COMPANY, A CORPORATIGN OF PENNSYLVANIA TUNGSTEN-TANTALUM ALLOY No Drawing.

This invention relatcs to filaments for incandescent lamps, electron discharge devices and similar devices, employing a filament which operates at high incandescence. More particularly, it relates to a filament composed principally of tungsten having a low rate of evaporation.

In incandescent lamps tungsten is used almost universally as the filamentary material since it has the lowest rate of evaporation of those metals which are sufliciently refractory to withstand lamp filament temperatures. Tungsten, however, gradually evaporates or volatilizes during the operation of the filament until finally the filament breaks due to the weakening caused by the vaporization of the tungsten. However, before the burn-out life of the lamp has been reached, the operation of the lamp usually has become so ineflicient that it is necessary to replace the lamp, due to blackening of the envelope by the vaporization of the filamentary material and volatilization of the impurities contained therein, such as carbon. The discolorization of the bulb, due to impurities contained in the filament, is relatively slight and takes place mainly during the initial burning of the filament. The discoloration is mainly dependent upon the vapor pressure of the filamentary material and-exists throughout the life of the lamp.

One of the objects of the present invention is, therefore, to produce filaments or other bodies of tungsten which have a materially lower rate of evaporation than ordinary tungsten. Y

Another object is to produce a filament composed primarily of tungsten which, when operated at high temperatures, will have a long life and will constitute a highly eflicient light source.

Another object is to provide a method of producing solid solution alloys of refractory metals.

Other objects and advantages will hereinafter appear.

I have found that when tantalum is in corporated with tungsten in small proportions, as a solid solution alloy, that the rate of evaporation of the filament is very ma- Application filed May 28, 1927. Serial No. 195,145.

terially reduced, blackening of the bulb largely prevented and the life and elliciency of the lamp increased. Tantalum, next to tungsten is the most refractory ot' the known metals and consequently alloys of tungsten and tantalum are the most refractory alloys producible. They are, therefore, particularly suitable for incandescent filaments for lamps, radio tubes, X-ray tubes, etc. Solid solution alloys of tungsten and tantalum may be operated for a given life at even higher temperatures than pure tungsten, due to the lower vapor pressure of the alloy and to the fact that such alloys have a melting point not appreciably lower than pure tungsten.

Heretotore, it has been proposed to manufacture alloys of tantalum and tungsten but the methods employed do notlend themselves to the production of solid solution alloys and it is only by virtue of such solid solution alloys that the vapor pressure of the tungsten is reduced.

I have found, in order to produce a solid solution alloy of tungsten and tantalum, that it is necessary to obtain an extremely intimate mixture of the powdered constitutents, to employ such powdered materials in a very fine state of subdivision and to heat treat the mixture for a long period of time under predetermined conditions to cause a transition or di-tfusion of the tungsten and tantalum into each other. When employing the ordinary commercial powdered oxides of the metals, a sufliciently intimate association of the materials cannot be obtained, due to the large particle size of the oxide grains. A mechanical mixture of such material, after reduction of the oxide, heat treated under most favorable conditions for over five hours failed to produce thorough diffusion of the tantalum into the tungsten.

In accordance with my invention I first produce an intimate mixture of the oxides of the constituent metals in an extremely fine state of subdivision by simultaneously precipitating such oxides from a common solution. The oxides are then reduced by means of carbon or carbon and hydrogen and a prolonged treatmcnt in hydrogen is given to the reduced material to eliminate the carbon therefrom. This carbon elimination is extremely important since carbon renders the slug, which is subsequently produced, extremely difficult if not impossible to work. The pu'rified and reduced powders are then pressed in a slug of convenient size an the same heat treated for a long period of time in vacuum to eliminate the gases therefrom, render them capable of being swaged and drawn and to cause a diffusion of the tantalum into the tungsten to produce a solid solution alloy.

. duce the desired proportion of tantalum in the ultimatemetal mixture. In the production of tungsten-tantalum alloys for lowering the rate of evaporation of the tungsten I prefer to employ from to 1 of tantalum. The mixture of the potassium tungstate and potassium tantalate solutions is heated to the boiling point and hot concentrated hydrochloric acid containing about 5% of nitric acid is added in an excess of about 10% to the amount necessary to neutralize the'excess potassium hydroxide and to precipitate the dissolved tantalum and tungsten oxides. The. precipitate of tantalum and tungsten, oxides thus produced, is

in a very fine state of subdivision and due to the simultaneous precipitation, an extremely intimate mixture is obtained. The precipitate is washed thoroughly, dried, ignited and finely powdered so as to pass througha 200 mesh screen.

This finely powdered and intimately associated mixture of oxides is then reduced in any convenient manner. one of the methods which I have used being as follows. I add to the powdered mixture from .15% to .4.% of carbon which may be in the form of lamp black. The material is then placed in a furnace and the temperature of the furnace gradually raised from zero to about 800 C. An

atmosphere of illuminating gas is maintained about the powdered materials to assist. in the reduction of the oxides to the metals. A period of approximately three hours is consumed in bringing the furnace up to the temperature specified which should be sufficient to entirely convert the oxide either to the metal or the carbide thereof. An atmosphere of pure dry hydrogen is then passed through the furnace and the temperature increased over a period of about 1% hours up tofrom 1200 to 1400 C. which temperature is then maintained for about five hours. This high temperature heating in hydrogen is for the purpose of completely eliminating the carbon and carbides from .the powdered material since the carbon in the finished slug renders the metal extremely diflicult to work.

The reduced and purified mixture of tantalum and tungsten is then molded under pressure into slugs of suitable proportions for treating. The sizes which I employed were one-quarter inch square and about eight inches long. These slugs were placed in a vacuum furnace and slowly heated up to around 1200 C., the vacuum pumps being maintained in operation so as to remove from the furnace all gases liberated from the slug. The temperature of the furnace should be increased at such a rate as to enable a vacuum of about .1 millimeters to be: maintained. With the apparatus employed the maximum temperature of about 1200 was obtained in about two hours.

The sintered slug may then be removed from the vacuum furnace, placed in a vacuum treating bottle and its temperature slowly raised upto about 90 to 95% of the fusion temperature of the slug. This temperature is maintained for a considerable period of time, that is, from 20 to 30 minutes to cause the tantalum to thoroughly diffuse intothe tungsten. The increase in tem erature should be effected at a sufficiently s ow rate to enable a vacuum of about .1 millimeters to be maintained in the treating bottle. ploying a one-quarter inch square slug I have obtained satisfactory treating by first passing a current of about 250 amperes through the slug, maintaining this current until no further gases were given off by the slug as indicated by the vacuum gage connected to the treating bottle, then raising the voltage so as to increase the current to 300 amperes, causing more gas to be liberated, maintaining this current until no further gas was given off and thus progressively increasing the current by steps of 50 amperes until a temperature corresponding to around 90% or 95% of the fusion temperature of the slug was obtained. This temperature was found to be around 1200 to 1250 C. in vacuum and should be maintained for a sufficient period to cause the tantalum to thoroughly diffuse into and form a solid solution with the tungsten. A period of from 20 to 30 minutes was found to be sufficient. The treated slug may then be swaged and drawn to filamentary size in the manner ordinarily employed for pure tungsten.

Tungsten-tantalum alloys containing from .75 to 1.5% tantalum by weight prepared in accordance with the foregoing method when the cross section thereof was etched to show the crystal formation indicated that a true solid solution was obtained.

\Vhile this processhas been described with particular reference to the production of an alloy of tungsten and tantalum, it is to be understood, of course, that it may also be applied to other metals, such as molybdenum. zirconium, uranium, etc. It is further understood that many modifications and changes may be made in the process without departing from the invention.

\Vhat is claimed is:

1. The method of forming solid solution alloys of tungsten and tantalum which comc r prises forming an intimate admixture of tungsten and tantalum oxides in the desired proportion. carbonizingsaid oxides, reducing said carbonized material to metal, compacting the metal powder and sintering the compacted metal powders to near fusion in vacuo.

2. The method of forming solid solution alloys of tungsten and tantalum which comprises forming an intimate admixture of tungsten and tantalum oxides in the desired proportions, effecting a carbonization of said admixed oxides, reducing the carbides with hydrogen, compacting the metal powders thus obtained. and sintering the compacted metal powders to near fusion in vacuo. 3. The method of forming a solid solution alloy of tungsten and tantalum which comprises intimately admixing tungsten oxide with tantalum oxide. the tantalum oxide in amounts sutlicient to give not more than 1.5 per cent tantalum; carbonizing said oxides, reducing the carbonized material by heating in pure dry hydrogen. agglomerating the metal powders. and effecting a consolidation thereof by heat treatment to near fusion in a substantial vacuo.

4. The method of forming solid solution alloys of tungsten and tantalum which comprises intimately admixing tantalum oxide with tungsten oxide in the proportion to give between .75 per cent and 1.5 per cent tantalum in the final product. carlonizing said mixed oxides. reducing the carbide to metal, agglomerating the metal powders, and consolidating the compacted article by heat treatment to near fusion in vacuo.

5. The method for forming a solid solution alloy of tungsten and tantalum which comprises torming an intimate admixture of tantalum and tungsten oxides in the proportion to give between .75 and 1.5 per cent tantalum in the final product, carbonizing said oxide compounds, reducing the carbides in pure dry hydrogen, agglomerating the metal powder, and consolidating by heat- 69 treatment to near fusion in vacuo.

In testimony whereof, I have hereunto subscribed my name this 27th day of May 1927.

JOHN H. RAMAGE. 

